UPDATE updates a master file by applying modifications
from one or more transaction files.

UPDATE shares the bulk of its syntax with other PSPP commands for
combining multiple data files. See Combining Files Common Syntax,
above, for an explanation of this common syntax.

At least two FILE subcommands must be specified. The first FILE
subcommand names the master file, and the rest name transaction files.
Every input file must either be sorted on the variables named on the
BY subcommand, or the SORT subcommand must be used just after the FILE
subcommand for that input file.

UPDATE uses the variables specified on the BY subcommand, which is
required, to attempt to match each case in a transaction file with a
case in the master file:

When a match is found, then the values of the variables present in the
transaction file replace those variables’ values in the new active
file. If there are matching cases in more than more transaction file,
PSPP applies the replacements from the first transaction file, then
from the second transaction file, and so on. Similarly, if a single
transaction file has cases with duplicate BY values, then those are
applied in order to the master file.

When a variable in a transaction file has a missing value or when a string
variable’s value is all blanks, that value is never used to update the
master file.

If a case in the master file has no matching case in any transaction
file, then it is copied unchanged to the output.

If a case in a transaction file has no matching case in the master
file, then it causes a new case to be added to the output, initialized
from the values in the transaction file.